The current trend in acoustic transducer technology has been toward smaller microphones. Currently, electret microphones based on thin, charge-carrying membranes have been used in most applications. However, these microphones suffer from degradation after exposure to high temperatures. Capacitive MEMS microphones are gaining popularity because they can withstand the high temperatures experienced during solder-reflow and, therefore, reduce manufacturing cost. Piezoelectric MEMS microphones have been researched for more than 30 years and can potentially combine the advantages of electret microphones and MEMS capacitive microphones in a cost-effective manner. Unfortunately, piezoelectric MEMS microphones traditionally suffer from high noise floors due, in part, to diaphragm tension caused by residual stress in thin films. For example, diaphragm microphones are constrained on all edges, which leads to high diaphragm tension that results in decreased sensitivity. Conventional cantilevered designs, such as rectangular cantilever beam microphones, also suffer from the effects of residual stress despite being substantially released from the surrounding substrate; instead, the small amount of residual stress causes the cantilever to bend away from the substrate plane, either upwards or downwards. This bending causes the gap around the cantilever to increase, decreasing the acoustic resistance and resulting in an undesirable decrease in low-frequency sensitivity.
Thus, there is a need in the piezoelectric MEMS acoustic transducer field to create a new and useful acoustic transducer with low frequency sensitivity despite residual stresses.